Referring to FIG. 1, it is common to lag or insulate the attic or loft space of a building by laying an insulation material 2, usually a mineral wool, between the parallel joists 4. It is also known to add a further layer of insulation in the transverse direction to further reduce heat loss. A problem with adding this further layer is that it is no longer clear where the underlying joists are, and, therefore where it is safe to store items or walk on. Thus, there exists the danger that items and people could fall through the plasterboard between the joists.